Heat exchangers are utilized in a variety of industrial processes to transfer heat between two or more fluids by indirect heat exchange. There are different designs for heat exchangers. For instance, one heat exchanger design is commonly referred to as a shell and tube design in which one fluid flows through the tubes and another fluid flows outside the tubes but inside a shell housing that retains the tubes. The fluid can be a liquid, vapor or a combination thereof. Further, the shell can be formed by or integrated with other equipment in which the heat transfer is to be conducted, for instance, a distillation column.
In another type of design, known as a plate-fin heat exchanger, a series of plates, referred to commonly as parting sheets, are connected at their respective edges by end bars and fins to enhance the heat transfer between the plates. Header tanks connected to the plates introduce the process and/or working fluids into the passages formed between the plates to accomplish the indirect heat exchange between the fluids.
Where one of the fluids is a liquid to be boiled at a boiling side surface of the heat exchanger, a porous coating can be used along the boiling side surface to promote heat transfer through a given surface, per unit surface area, (i.e., heat flux) in response to a given temperature difference at which boiling of the fluid will occur. For example, U.S. Pat. No. 4,917,960, discloses a coating that is formed from an aqueous solution containing a binder, such as a mixture of chromates and phosphates; and a fugitive or transient pore forming material such as a polyester; and aluminum particles. The coating can be applied as a slurry onto the surface of a heat exchange surface. Removal of the fugitive layer by heat or chemical solvent forms the resultant porous layer. The resulting porous coating can have a porosity ranging between 20 percent and 90 percent. The pore size can range between 20 microns and 60 microns. The aluminum particles of the slurry can have an average diameter of less than 4 microns.
Heat transfer efficiency is generally used to assess the performance of the porous coatings. As used herein and throughout the specification, the performance is defined by a temperature difference, ΔT, that is equal to T1−T2, where T1 is defined as the temperature of the working fluid and T2 is defined as the temperature of the process fluid to be heated to its predetermined temperature (e.g., boiling point). A coating with a relatively lower ΔT would be considered better performing, by virtue of its ability to promote greater heat transfer to the process fluid for a given input of heat source (e.g., a gas located on the shell side of a shell and tube heat exchanger design having a temperature greater than that of the process fluid flowing within the tube of the shell and tube heat exchanger). Improved performance of a coating is defined at least in part by a reduction in the ΔT. As will be explained in greater detail below, heat transfer efficiency will be used to assess coating performance of porous coatings for various applications, including boiling heat transfer applications, whereby heat is transferred from a heat source to a fluid to cause it to boil.
Generally speaking, the coatings disclosed in U.S. Pat. No. 4,917,960 are representative of conventional coatings that suffer from an unacceptably high ΔT. In other words, a large amount of heat energy is required to be transferred to the boiling surface to boil the process fluid, which translates into inefficient processes having excessive power consumption.
Current available methods for applying powder and/or slurry compositions along the inner surface of heat exchanger conduits (e.g., tubes) can produce significant variation in the resultant porous coating thickness, which results in unacceptable performance variation. For example, current methods tend to produce coating defects along the inner surface of heat exchanger tubes such as “blow holes” in FIG. 6a and “slumping” in FIG. 6e and many others (e.g., poor bonding in FIG. 6b, bare spots in FIG. 6c, delamination in FIG. 6f and flaking in FIG. 6d) as also shown in FIGS. 6a-6f, any of which ultimately can reduce and/or degrade performance of the porous coated boiling surface.
Today's methods for applying powder and/or slurry compositions cannot reliably produce consistent porous coatings. For example, conventional spray methods cannot produce porous coatings with uniform thickness along heat exchanger surfaces. The problem is even more challenging when the porous coating is applied along the surface of an inner diameter of a conduit or tubular structure, which tends to be difficult to coat. As the inner diameter of heat exchanger conduits become smaller with emerging applications, coating consistency and thickness uniformity therealong becomes increasingly difficult to achieve with current available methods, and in many instances, may not be possible. The end result is heat transfer applications which are inefficient, and require increased power consumption to operate. In view of these shortcomings, there remains an unmet need for improved methods for applying slurries and/or powder compositions to produce consistent porous coating compositions in a controlled and reproducible manner.